1. Field
The present application generally relates to manufacturing processes of semiconductor devices and more specifically to laser, or other light absorption based, spike annealing of structures on a substrate.
2. Related Art
Continued demand for smaller, more compact, faster, and more powerful chips forces the device geometries to scale down to and beyond the 100 nm node. Such aggressive downscaling in device geometries increases the short channel effects. This reduces the differentiation between Ion (on state device current which is dependent on device type) and Ioff (off state device current or leakage currents), which reduction is essential for maintaining the device functionality. Thus the critical challenge in scaling device geometries is to maintain a distinction between Ion and Ioff.
Typical spike anneal is performed by subjecting a semiconductor substrate having implanted dopants to temperature treatment in a rapid thermal processing (RTP) system. The typical annealing profile using RTP involves ramping up to a target temperature, e.g. 1060° C., soaking the substrate at the target temperature for a period of time (soak time), and ramping down to a base temperature, e.g. 200° C. For spike anneal, high ramp rates, e.g., 10,000° C./sec or higher, and shorter than 1 sec or no soak time are desired to prevent excessive dopant diffusion. In addition to the tight temperature control requirement, gas composition in the annealing ambient can also need to be controlled. For example, the presence of oxygen has been found to be necessary in order to decrease the evaporation or out-diffusion of implanted dopants such as boron and arsenic, but too much oxygen in the annealing ambient results in oxygen modified diffusion (OED) and limits the creation of shallow junctions, particularly when dopants such as boron are used.
Current technology involves ion implantation followed by a rapid thermal spike annealing process. The main parameters in any spike annealing process are the peak temperature, and dwell time. A measure of spike sharpness, tR, is defined as the time spent by the substrate within 50° C. of peak temperature. Higher peak temperature has the primary effect of causing increased dopant activation, hence causing reduced residence time and increased Ion. Different devices have different requirements of dopant activation and hence different choices for peak temperature. For the same peak temperature, an increase in dwell time has the primary effect of increasing dopant diffusion, hence increasing the leakage currents. A common technology has made use of a CO2 laser (10.6 μm wavelength). Because of the far infrared (FIR) wavelength, absorption is induced only by free carrier generation, which requires either heavily doped substrates or thermal carrier generation to create the free carriers. With regards to heavily doped silicon substrates, resistivity between ˜0.005 to ˜0.020 ohm-cm for p-type is needed to induce free-carrier absorption by exposure to a CO2 laser (10.6 μm wavelength). With regards to thermal carrier generation, temperatures around 400° C. are typically required to induce free-carrier absorption by exposure to a CO2 laser (10.6 μm wavelength). This temperature could be established by a traditional resistance based hot plate or by using a second pre-heat laser (to heat local area in front of the scanning CO2 laser beam. Because of the absorption properties of the locally doped or pre-heated surface, the CO2 anneal produces a localized heating within only the top one-third of the substrate, which leaves the bottom two-thirds of the substrate essentially as a cold sink. This cold sink allows for very quick thermal quenching of the elevated temperature induced by the laser exposure, thus, the millisecond bake process.
The main effort behind spike anneal for current ion implant technology is to reduce dwell time without compromising on the required level of dopant activation. When looking to apply similar spike anneal processes to photolithography resist chemistry applications, desirable in the art are methods and systems that can achieve the peak temperature at a short controllable dwell time without making use of heavily doped substrates to get absorption or without making use of pre-heat thermal treatments that would thermally de-protect or thermally degrade resist chemistries (when using traditional resistance based hot plate at long times) or in the case of a pre-heat laser beam process, raise the temperature of the local substrate higher than the target spike peak temperature for resist applications.